Why should you only reattach after at least 30 minutes? What happens when you don't wait 30 minutes and what's the danger of it?
When your transaction is younger than 30 minutes, there is a significant chance that other transactions will use it as a tip for their transaction (and therefore confirm it). So, first, if you reattach early, you are wasting your PoW resources.
To make matters worse, when you reattach really early (maybe even multiple times), there are two transactions on the tangle (perhaps even different ones of them arrive first on different nodes) that cannot both be confirmed, but some nodes might not know that already. So it can happen that some transactions choose to verify your original transaction, and some choose to verify the reattachment. At some point, the network (or the coordinator) will decide which of your transactions will get confirmed, and every transaction who tried to confirm the other one is essentially lost (will never confirm and need reattachment). Therefore, by reattaching too early, you will inevitably create "time-bombs" in the tangle which will force others to reattach more often, too.
To be fair, the tangle is able to handle some kind of "misuse" (be it deliberate or not), but if all users started to reattach their transactions immediately, it would surely be noticable.
So, be a nice neighbor and just don't do it :-)
The reason you are told to wait 30-60 minutes before reattaching is that your original transaction has a good chance of being confirmed within 30-60 minutes. Reattaching is more for when your transaction has gotten looked over so much that it's no longer fresh and clients are not looking to confirm it.
Reattaching in less than 30-60 minutes is just like hitting the close elevator button, it doesn't help and is wasted your resources and time.